Metal cap apparatus and method

ABSTRACT

A method of forming a metal layer may include forming an opening in a substrate; forming a liner over sidewalls of the opening; filling the opening with a first metal; etching a top surface of the first metal to form a recessed top surface below a top surface of the substrate; and exposing the recessed top surface of the first metal to a solution, the solution containing a second metal different from the first metal, the exposing causing the recessed top surface of the first metal to attract the second metal to form a cap layer over the recessed top surface of the first metal.

PRIORITY CLAIM AND CROSS-REFERENCE

The present application claims priority to and is a continuation of U.S.patent application Ser. No. 13/849,608, filed on Mar. 25, 2013, andentitled “Metal Cap Apparatus and Method,” which application claims thebenefit of U.S. Provisional Application No. 61/778,307, filed on Mar.12, 2013, entitled “Metal Cap Apparatus and Method,” which applicationsare hereby incorporated herein by reference.

BACKGROUND

As modern integrated circuits shrink in size, the associated featuresshrink in size as well. As transistors shrink, features such as throughvias and other interconnect elements shrink in size as well. In manyinstances, various layers of circuit on chips, dies, in packages, onPCBs and other substrates are interconnected between various layers byway of vias. Vias can be formed in openings through a substrate filledwith a conductive metal. Typically, the vias are connected to traces orother conductive structures to permit non-aligned contact points indifferent layers to be connected.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1-6 are cross-sectional diagrams of illustrating intermediatesteps in a process for capping a via according to various embodiments;

FIGS. 7A-7C are cross-sectional diagrams illustrating shapes of via capsformed according to embodiments; and

FIG. 8 is a flow diagram illustrating a method of forming a via capaccording to embodiments.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the illustrative embodiments are discussed indetail below. It should be appreciated, however, that the presentdisclosure provides many applicable concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the embodimentsof the disclosure, and do not limit the scope of the disclosure.

The present disclosure is described with respect to embodiments in aspecific context, namely forming metal caps on recessed features usingan electroless chemical reaction, and in particular, forming metal capson through vias. The embodiments of the disclosure may also be applied,however, to a variety of features, plating scenarios or other metalfeature formation techniques. Hereinafter, various embodiments will beexplained in detail with reference to the accompanying drawings.

Conductive features are commonly used in semiconductor device andpackaging fabrication to connect active devices such as transistors, toprovide interconnects or mounting pads for package-on-package mountingsolder balls, or the like. In some instances, a metal via may bedisposed through one or more dielectric layers in interlayer dielectrics(ILDs) or intermetal dielectrics (IMDs). In other instances, vias may beused through substrates or similar structures. Vias may be formed byelectroplating, deposition in a damascene or masking processes, or byother techniques. Copper is a commonly used metal for vias andinterconnects due to its cost, predictable properties, and adhesion tomany different substrates. Other metals used in vias may include gold,aluminum, tungsten, cobalt, palladium, nickel, silver, compounds oralloys of the same, or other conductive materials.

In some instances, caps may be used to cover the surfaces of vias,particularly over vias, which may be prone to corrosion, contaminationor surface imperfections arising from, for example, steps of the deviceor package fabrication process. Via caps may also be used to plate adissimilar metal via to increase adhesion of subsequent layers orstructures, or to alter the surface electrical properties of the via.For example, cobalt may be used to cap a copper via to provideresistance to corrosion and to prevent copper leaching into subsequentlayers.

In an embodiment, a metal cap may be formed within the via opening byreplacing a portion of the via metal, allowing the cap material toextend from the top of the via opening down into the via opening,preventing exposure of the via material, particularly at the top surfaceof the via.

FIG. 1 illustrates an initial via 108 structure for applying a capaccording to an embodiment. In an embodiment, the substrate 102 may be asemiconductor such as silicon, or may be a dielectric such as a nitride,oxide or the like. Furthermore, the substrate 102 may be a redirectionlayer, IMD, ILD passivation layer, or any other nonconductive structure.The substrate 102 may have a via opening 104 formed therein. While thesidewalls of the via opening 104 are illustrated as being substantiallylinear and perpendicular to the top surface 112 of the substrate 102,the sidewalls may be at an angle greater or less than 90 degrees to thesubstrate 102 top surface 112, and may be curved, in linear sections, orotherwise non-linear. Additionally, while the via opening 104 of FIG. 1is illustrated as extending through the substrate 102, the embodimentspresented herein may be applied to a recess or opening extendingpartially through the substrate 102 as well.

A via 108 may be disposed within the via opening 104, and may have a viatop surface 110 that extends to near the substrate top surface 112. Inan embodiment, the via top surface 110 and the substrate top surface 112may be substantially level, may optionally be substantially planar. Thevia opening 104 may have one or more liners 106 that may comprise a baselayer, isolation layer or protective layers disposed on the sidewalls.For example, in an embodiment, the liner 106 may be a metal such aspalladium, which may prevent a diffusion of a metal such as copper fromdiffusing from the via 108 into the substrate 102. The liner 106 mayalso, in an embodiment, have an isolation layer such as a high-k nitrideor oxide to electrically isolate the via 108 from the substrate 102.

It has been discovered that forming a cap on the top surface 110 of thevia 108, with the cap lying on or above the top surface 112 of thesubstrate 102 may lead to undesirable effects. These effects may beparticularly prominent where the overlying cap is formed using chemicalvapor deposition (CVD) processes. An overlying cap, for example, maycause via 108 material leakage, current shorts across adjacent vias 108and non-uniform caps. For example, a deposited cap may be imperfectlyformed, and may not completely cover the via 108 at the edges of thevia. In such a structure, the surface 110 of the via 108 may be exposed,permitting diffusion of via 108 material into an overlying dielectriclayer. Additionally, CVD deposition of metal cap layers over the via 108may result in residual material deposited between the caps of adjacentvias 108, resulting in shorts across the vias 108.

An embodiment may comprise a via 108 with a metal cap layer formedinside the via opening 104. An electroless chemical process such aselectroless deposition or metal exchange process may provide for a moreuniform deposition coating than an electroplating process by avoidingcurrent density uniformity problems. In an embodiment, the metal cap maybe formed using an electroless metal exchange process. In such aprocess, an upper portion of the via 108 metal may be replaced with asecond metal without the use of a voltage or outside electrical powersource. In such an embodiment, the substrate 102 and via 108 may bedisposed in a metal solution 114 having ionic metal (M⁺) dissolvedtherein. A metal via 108 comprising a metal having a lower reductionpotential will oxidize in a reduction-oxidation reaction when exposed toan ionic metal having a higher reduction potential. The ionic metal willundergo reduction and exchanging places in the via 108 with the originalvia 108 metal. In an embodiment, a copper via 108 may be capped with adifferent metal such as gold, platinum, silver or ruthenium, each ofwhich has a reduction potential greater than that of copper. It shouldbe noted that other metals, alloys or materials having higher reductionpotentials may also be used. Thus, the metal solution 114 may have oneor more ionic metals (M⁺) dissolved therein, with the ionic metal (M⁺)having a reduction potential greater than the material of the metal via108. Equations 1-5 illustrate the reduction potentials of materials thatmay be used in embodiments:Cu²⁺+2e ⁻

Cu(s)(0.34v)  1)Pt²⁺+2e ⁻

Pt(s)(1.2v)  2)Ag⁺ +e ⁻

Ag(s)(0.8v)  3)Ru²⁺+2e ⁻

Ru(s)(0.8v)  4)Au³⁺+3e ⁻

Au(s)(1.52v)  5)

FIG. 2 illustrates a via structure having an initial cap 202 layerformed thereon according to an embodiment. The cap 202 is formed througha metal exchange process, with the ionic metal (M⁺) displacing materialof the via 108. The via 108 metal becomes a second ionic metal (N⁺) inthe metal solution 114. The cap 202 has a top cap surface 204 that isabout the same level as the original via top surface 110 (in FIG. 1),with the metal cap surface 110 moving down as the cap 202 displaces thematerial of the via 108. While the cap 202 is illustrated as being adistinct layer separate from the via 108, it should be understood thatin an embodiment, the cap layer may be diffused into the via material,the top surface 10 of the via 108 indicating a transition between amajority via 108 metal and a majority cap 202 metal.

Additionally, the liner 106 in the via opening 104 may be formed of amaterial that would not react with the metal ions (M⁺) of the metalsolution 114. For example, the liner 106 may be a high-k dielectric thatwould not react in a reduction-oxidation reaction with the metal ion(M⁺) of the metal solution 114. In another embodiment, the liner 106 maybe a metal that has a higher reduction potential such the material inthe metal solution 114 would not react in a reduction-oxidation reactionwith the metal ion (M⁺) of the metal solution 114. For example a via 108may be copper, the liner 106 may be palladium (See Eq. 6), and the metalion (M⁺) may be silver. The reduction potential of palladium isillustrated in equation 6, below:Pd²⁺+2e ⁻

Pd(s)(0.915v)  (6)

The reduction potential of silver is less than palladium, but greaterthan copper, so the silver would reduce the copper and not thepalladium, resulting in the palladium liner 106 remaining intact while asilver cap 202 is formed on the copper via 108. In an embodiment, thecap formation process may be achieved as an electroless process, as themetal exchange reaction described above occurs without application ofvoltage.

FIG. 3 illustrates a via structure having a cap 202 layer formed thereonaccording to an embodiment. The top surface 110 of the via 108 migratesdown as the cap 202 grows, causing the cap 202 to grow thicker while thecap top surface 204 remains about level or planar with the top surfaceof the substrate 102. The thickness of the cap 202 may be controlled bylimiting the duration of the period that the metal solution 114 is incontact with the via 108 and cap 202. In an embodiment, the cap 202 maybe formed to a thickness of about 20 nm or less.

The shape of the via top surface 110 may be controlled by introducingone or more materials into the metal solution 114. FIG. 7A shows a domedvia top surface 110, FIG. 7B shows a cupped via top surface 110, andFIG. 7C shows a flat via top surface 110. The different shapes of thevia top surface 110 underneath the cap 202 may be controlled byintroducing a material such as carbon, oxygen, hydrogen, nitrogen,sulfur, chlorine or bromine into the metal solution 114. For example, ametal solution 114 with carbon may be used to control the shape of thevia top surface 110. In such an embodiment, a carbon concentrationhigher than the exchanged metal from the via 108 present in the metalsolution may result in a cupped via top surface 100, as shown in FIG.7A. A metal solution with no carbon may result in a cupped via topsurface 110 is formed, as shown in FIG. 7B. A carbon concentration lessthan the exchanged metal from the via 108 present in the metal solutionmay result in the flat via top surface 110, as shown in FIG. 7C.

FIG. 4 illustrates an initial via 108 structure for applying a capaccording to another embodiment. The substrate 102, via 108 and optionalliner 106 may be formed using similar processes and/or materials asdiscussed above. In such an embodiment, the via 108 may be etched backand a cap 202 formed by way of an electroless chemical deposition asdiscussed in greater detail below. A substrate 102 with a via 104 andoptional liner 106 may be provided.

FIG. 5 illustrates etchback of the via 108 according to an embodiment.The via 108 may, in an embodiment, be etched to a predetermined depthby, for example, a selective etch. The thickness of the cap 202 may becontrolled by etching the via 108 back to a depth equal to apredetermined cap thickness. In one embodiment, the via 108 may beetched back by about 20 nm or less. The via top surface 110 may beetched to form a shape as shown in FIGS. 7A-7C. The etchant used to etchback the via 108 may be adjusted to result in a desired via top surface110. For example, in an embodiment, the via 108 may be etched back withan etchant such as CH₄, C₄F₈, CHF₃, CH₂F₂ or Cl₂. The ratio of carbon tochlorine or fluorine may be used to control the shape of the top surface110. For example, the domed via top surface 110, as shown in FIG. 7A maybe formed by etching with an etchant having a greater percentage ofcarbon than of chlorine or fluorine. The cupped via top surface 110, asshown in FIG. 7B may be formed by etching with a chlorine or fluorinebased etchant having no carbon. The flat via top surface 110, as shownin FIG. 7C may be formed by etching with an etchant having a lowerpercentage of carbon than of chlorine or fluorine.

FIG. 6 illustrates a cross sectional view of deposition of a cap 202over a via 108 in a via opening 104 according to an embodiment. In anembodiment a cap 202 may be deposited, for example, using a metalsolution comprising hypophosphite or dimethlyaminoborane (DMAB). In suchan embodiment, a hypophosphite/metal solution or DMAB/metal solution maybe applied to an etched via 108 to form the cap 202 in the recesscreated by etching back the via 108. Multiple metal materials may bedeposited using such techniques, and particularly, metals having a lowerreduction potential than the via 108 material. For example,hypophosphite may be used with to form a cobalt and tungsten cap 202according to equations 7-9, below:Deposit Co: Co²⁺+2H₂P0₂ ⁻+4OH⁻=Co_((s))+2HP0₃ ²⁻+H₂+2H₂O  7)Co-Deposit P: 4H₂P0₂ ⁻=2P_((s))+2HP0₃ ²⁻+H₂+2H₂O  8)Co-Deposit W: WO₂ ²⁺+6H₂P0₂ ⁻+2OH⁻+2H₂O=W_((s))+6H₂P0₃ ⁻+3H₂  9)

In another embodiment, DMAB may be used to form a cobalt and tungstencap 202 according to equations 10-12, below:Deposit Co:3Co²⁺+2(CH₃)₂NH:BH₃+6OH⁻=3Co_((s))+2B(OH)₃+3H₂+2(CH₃)₂NH  10)Co-Deposit B: 4(CH₃)₂NH:BH₃+6H₂O=2B_((s))+2B(OH)₃+9H₂+4(CH₃)₂NH  11)Co-Deposit W: WO₂²⁺+4(CH₃)₂NH:BH₃+4OH⁻+2H₂O=W_((s))+2B(OH)₃+2H₂+2(CH₃)₂NH  12)

Hypophosphite or DMAB may be used to facilitate the reaction, creatingan autocatalytic reduction-oxidation reaction, with the surface of thevia 108 attracting the plating to fill the recess etched in the via 108.Phosphorus or boron may be co-deposited with cobalt and tungsten in suchembodiment as a byproduct of the reaction. However, other co-depositmaterials may include carbon, oxygen, hydrogen, nitrogen sulfur,chlorine or bromine, depending on the materials deposited and theadditives in the metal plating solution.

FIG. 8 is a flow diagram illustrating a method 800 of forming anelectroless via cap 202 according to embodiments. One or more vias 108may be formed in the substrate 102. The vias may be formed in block 802by electroplating, masking and deposition using a technique such as CVD,or another suitable technique. A cap 202 may be formed over the via 108,and within the via opening 104, in block 804. The cap 202 may optionallybe applied in block 804 optionally by the metal exchange process or bythe etching/electroless deposition process. The metal exchange processmay be performed by applying a metal solution 114 to the via 202 inblock 806. The metal exchange may be performed in block 808, retainingthe metal solution 114 in contact with the via 108 and cap 202 until thecap 202 reaches a predetermined thickness. The etching/electrolessdeposition process may be performed by etching the via to apredetermined thickness in block 810. A metal/hypophosphite solution maybe applied to the via 108 in block 812, or a metal/DMAB solution may beapplied to the via 108 in block 814. The metal/hypophosphite ormetal/DMAB solution may be retained in contact with the via and cap 202until the recess formed by etching the via 108 is filled. In anembodiment, any excess cap 202 material may be planarized to bring thetop surface 204 of the cap 202 to about the same level with thesubstrate stop surface 112, for example, by a CMP or the like.

Thus, according to an embodiment, a method of forming a metal layer maycomprise bringing a via into contact with metal solution, the viadisposed in an opening in a substrate, and forming a metal cap in theopening and in contact with the via, the metal cap formed by anelectroless chemical reaction. The method may further comprise applyinga metal solution to the via, with the metal solution forming the metalcap. The method may further comprise etching back the via below a topsurface of the substrate prior to forming the metal cap. The metalsolution may comprise at least cobalt and may further comprise tungsten.Forming the cap may comprise forming the cap to comprise at leastcobalt, and optionally to further comprise tungsten. The metal solutionmay comprise at least hypophosphite or dimethlyaminoborane.

Another embodiment of a method of forming a metal layer may compriseapplying a metal solution to a via disposed in an opening in asubstrate, the opening having a liner disposed on sidewalls of theopening, and the liner separating the substrate and the via. A metal capmay be formed in the opening and in contact with the via. The metal capmay be formed by a metal exchange reaction. The method may comprisebringing the metal solution into contact with a surface of the via thatis about level with a top surface of the substrate. The metal solutionmay comprise a first metal, and the via may a second metal differentfrom the first metal. The first metal may have a reduction potentialgreater than a reduction potential of the second metal. The first metalmay be at least one of gold, silver, ruthenium and platinum.

An apparatus according to an embodiment may comprise a substrate havingan opening disposed therein and extending from a first side into thesubstrate, a via disposed in the opening, the via comprising a firstmetal, and a cap disposed in the opening and covering a first surface ofthe via, the cap disposed between the first surface of the substrate andthe via. The cap may comprise at least a first metal and the via maycomprise at least a second metal different from the first metal. A linermay be disposed on a sidewall of the opening, the liner disposed betweenthe via and the substrate.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A method of forming a metal layer, the methodcomprising: forming an opening in a substrate; forming a liner oversidewalls of the opening; filling the opening with a first metal;etching a top surface of the first metal to form a recessed top surfacebelow a top surface of the substrate, the etching including exposing thetop surface of the first metal to an etchant wherein a concentration ofcarbon in the etchant relative to a concentration of chlorine orfluorine is selected based upon a desired cross-sectional shape of therecessed top surface; and exposing the recessed top surface of the firstmetal to a solution, the solution containing a second metal differentfrom the first metal, the exposing causing the recessed top surface ofthe first metal to attract the second metal to form a cap layer over therecessed top surface of the first metal.
 2. The method of claim 1,wherein the solution is selected from a group consisting of ahypophosphite/second metal solution, a dimethylaminoborane/second metalsolution, and combinations thereof.
 3. The method of claim 1, whereinthe second metal has a reduction potential lower than a reductionpotential of the first metal.
 4. The method of claim 3, wherein thesecond metal comprises at least one of tungsten or cobalt.
 5. The methodof claim 3, wherein the first metal comprises copper.
 6. The method ofclaim 1, wherein the etching the top surface of the first metal to formthe recessed top surface comprises exposing sidewalls of a portion ofthe liner.
 7. A method of forming a metal layer, the method comprising:exposing a top surface of a metal via to a metal solution, the metal viadisposed in a substrate, the top surface of the metal via beingsubstantially co-planar with a top surface of the substrate; anddisplacing a first metal of the metal via with a second metal of themetal solution to form a cap layer, the first metal of the metal viabeing displaced into the metal solution, wherein the displacing causesthe top surface of the metal via to migrate in a direction away from themetal solution, wherein an interface is formed between the top surfaceof the metal via and the cap layer; and controlling the cross-sectionalshape of the interface by introducing into the metal solution a materialselected from the group consisting of carbon, oxygen, hydrogen,nitrogen, sulfur, chlorine and bromine.
 8. The method of claim 7,wherein the first metal is different from the second metal.
 9. Themethod of claim 7, wherein a reduction potential of the first metal isless than a reduction potential of the second metal.
 10. The method ofclaim 7, wherein the displacing the first metal of the metal via withthe second metal of the metal solution causes the top surface of themetal via to have a predetermined shape, the predetermined shape beingdetermined by an amount of a first material in the metal solution,wherein the first material is carbon, oxygen, hydrogen, nitrogen,sulfur, chlorine or bromine.
 11. The method of claim 10, wherein thefirst material is carbon, and wherein a concentration of carbon in themetal solution is greater than a concentration of the displaced firstmetal in the metal solution.
 12. The method of claim 7, wherein a topsurface of the cap layer is substantially co-planar with the top surfaceof the substrate as the top surface of the metal via migrates in thedirection away from the metal solution.
 13. The method of claim 7,wherein a liner comprising a third metal is disposed between the metalvia and the substrate, and wherein displacing the first metal of themetal via with the second metal of the metal solution comprisesdisplacing the first metal and not the third metal.
 14. The method ofclaim 13, wherein a reduction potential of the third metal is greaterthan a reduction potential of the second metal.
 15. The method of claim13, wherein the first metal comprises copper, the second metal comprisessilver, and the third metal comprises palladium.
 16. The method of claim7, wherein the second metal comprises at least one of gold, silver,ruthenium or platinum.
 17. A method comprising: forming an opening in asubstrate; filling the opening with a first metal; etching a top surfaceof the first metal to form a recessed top surface below a top surface ofthe substrate by exposing the top surface of the first metal to anetchant wherein a concentration of carbon in the etchant relative to aconcentration of chlorine or fluorine is selected based upon a desiredcross-sectional shape of the recessed top surface; and exposing the topsurface of the first metal to a metal solution having an ionic metaldissolved therein to form a capping layer of a second metal over thefirst metal.
 18. The method of claim 17, wherein the top surface of thefirst metal is a first distance from the top surface of the substratebefore the exposing step and the top surface of the first metal is asecond distance from the top surface of the substrate after the exposingstep, the second distance being greater than the first distance.
 19. Themethod of claim 17, wherein the second metal has a higher reductionpotential than the first metal.